DENIERS SAY:

CO2 in the air comes from volcanoes, the oceans and other natural sources, not humans.

SCIENCE SAYS:

What's causing climate change? Human beings. Just ask 97% of the top climate scientists in the world.

How can scientists be sure that humans are increasing heat-trapping carbon dioxide? First, the carbon dioxide in the atmosphere has increased about 40 percent since humans started burning dirty energy like coal and oil on a wide scale. Second, the carbon from dirty energy has a unique chemical signature that allows scientists to tell it apart from other sources of carbon. Third, the amount of oxygen in the atmosphere is decreasing. It takes oxygen to burn trees or coal, and the more burning we do, the more oxygen we suck up. The evidence is inescapable: Humans ARE changing our climate.

Additional info from the Union of Concerned Scientists

Carbon dioxide (CO2) is the main heat-trapping gas largely responsible for most of the average warming over the past several decades. There are ways that scientists can tease apart the atmospheric concentration of CO2 to see how much of the CO2 is from natural sources and how much is from combusted fossil fuel sources.

The atmospheric concentration of CO2 has increased from a pre-industrial era (AD 1000-1750) concentration of approximately 280 parts per million (ppm) to around 392 ppm in 2011. Carbon dioxide from burning fossil fuels is chemically different than carbon dioxide that occurs naturally, so scientists can tell that fossil fuel emissions comprise the largest source of the increase since the pre-industrial era.

Here’s how scientists know. The same elements (i.e., same number of protons in the nucleus) with different mass numbers (arising from the different numbers of neutrons in the nucleus) are called isotopes. Each carbon molecule has six protons in the nucleus, but there are many different isotopes with varying numbers of neutrons in the nucleus. Carbon isotopes from different sources can be “lighter” (high negative value) or heavier (lower negative value). For example, carbon from the ocean has a value of “0” while carbon from fossil fuels ranges from -20 to -32. Atmospheric carbon has an average value of -5 to -9 and it is becoming “lighter” over time as “lighter” carbon from fossil fuels become more abundant in the atmosphere (Figure 1).